Use monotonic clock for performance.now() (facebook#33983)

Summary:
In facebook#32695, the `Performance.now()` implementation changed to use unix epoch timestamps instead of a monotonic clock.

This is problematic, because it means that performance measurements get skewed if the device clock changes between two measurements.

With this change, the clock is now monotonic (and the implementation stays consistent between platforms).

More details and repro steps can be found in [this issue](facebook#33977)
Closes facebook#33977

## Changelog

[General] [Fixed] - Use monotonic clock for performance.now()

Pull Request resolved: facebook#33983

Test Plan:
Run on iOS and Android:
```
const now = global.performance.now()
console.log(`${Platform.OS}: ${now}`)
```

Reviewed By: JoshuaGross, cipolleschi

Differential Revision: D37066999

Pulled By: dmitryrykun

fbshipit-source-id: 298547bf39faea1b025c17ff2d2e1a03f929865b

React/CxxBridge/RCTJSIExecutorRuntimeInstaller.mm

@@ -22,8 +22,14 @@
     bindNativeLogger(runtime, iosLoggingBinder);
 
     PerformanceNow iosPerformanceNowBinder = []() {
-      auto time = std::chrono::system_clock::now().time_since_epoch();
-      return std::chrono::duration_cast<std::chrono::milliseconds>(time).count();
+      auto time = std::chrono::steady_clock::now();
+      auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(
+                          time.time_since_epoch())
+                          .count();
+
+      constexpr double NANOSECONDS_IN_MILLISECOND = 1000000.0;
+
+      return duration / NANOSECONDS_IN_MILLISECOND;
     };
     bindNativePerformanceNow(runtime, iosPerformanceNowBinder);
 

ReactAndroid/src/main/jni/react/jni/NativeTime.cpp

@@ -12,8 +12,14 @@ namespace facebook {
 namespace react {
 
 double reactAndroidNativePerformanceNowHook() {
-  auto time = std::chrono::system_clock::now().time_since_epoch();
-  return std::chrono::duration_cast<std::chrono::milliseconds>(time).count();
+  auto time = std::chrono::steady_clock::now();
+  auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(
+                      time.time_since_epoch())
+                      .count();
+
+  constexpr double NANOSECONDS_IN_MILLISECOND = 1000000.0;
+
+  return duration / NANOSECONDS_IN_MILLISECOND;
 }
 
 } // namespace react